It is often required to separate and remove suspended particulates from an aerosol or mist in an industrial process stream. The particulates may be solid or liquid dispersions ranging from submicron to micron-level sizes. Examples include the separation of sulfuric acid mists from acid manufacturing streams or plasticizer mists from the manufacture of polyvinyl chloride products. Solid aerosols, such as smoke, are also candidates for separation.
Fiber bed mist eliminators have found wide industrial application in the removal of aerosols or mists from gas streams. These process streams usually contain particles smaller than about 3 microns. It is generally known that fibers made of various materials may be utilized to construct fiber beds for fiber bed mist eliminators. Fiber bed mist eliminators may achieve separation efficiencies of 99% or greater.
In practice, a gas stream containing a mist or aerosol is directed through a fiber bed. Mist particles coming into contact with individual fibers are collected thereon. Mist particles then coalesce into droplets or a fine liquid film. Liquid is forced through the bed toward the downstream portion of the fiber bed by the continuous gas stream wherein gravity urges the liquid to drain downward. The liquid is collected from the bottom of the unit and the affluent gas stream exits from the top. Solid particles may be similarly removed by the introduction of an atomized liquid into the gas stream prior to entry into the mist eliminator. The atomized liquid dissolves the solid particles and irrigates the fiber bed.
The engineering and economic goal of an efficient fiber bed mist eliminator is to maximize the removal of the liquid or solid component of the gas stream while minimizing the cost of doing so. Such variables as fiber bed geometry, thickness, density (or void fraction), effective surface area, fiber material, and process stream velocity, volume, and composition all have an effect on the pressure drop across the fiber bed. Maintenance of larger pressure drops are energy intensive and therefore more expensive. Similarly, replacement of a fiber bed can be costly due to process down time and capital outlay for new fiber beds.
In an effort toward lowering the pressure drop across the fiber bed, and/or increasing the lifetime of the fiber bed, a designer may try decreasing the fiber bed thickness or expanding the void fraction of the fiber bed by altering the fiber bed material or altogether changing fiber composition. These trade-offs usually come at the expense of lowered separation efficiencies. Thus, there is always a need for an improved fiber bed medium that enables high separation efficiency at satisfactorily low costs.
The fibers which comprise fiber beds have been made from a variety of materials. The materials includes metals (e.g., stainless steel) and glass. Fibers of polymeric materials such as polyester, polyvinyl chloride, polyethylene, and the like have also earned acceptance. The choice of material may depend upon the temperature and/or corrosivity of the process stream, as well as the physical and structural properties of the fiber bed comprised of the constituent material. Designers are therefore concerned with a fiber's diameter, its bulk density, fiber bed surface area, void fraction and behavior in a "wet" or liquid-laden state during the mist eliminator application. Alteration of any of these variables can greatly influence the overall efficiency of the mist eliminator.
Polymer fiber bed media has been traditionally produced via conventional extrusion or melt-blown methods. The resulting polymer is formed into a sheet or webbed mat. Woven fabrics are also employed. The fabrics may be pleated or calendared for increased fiber bed surface area, stress, or mechanical stability. However, most commercially available synthetic materials are limited in their application because their resistivity to chemical degradation is insufficient in many solvent-containing or corrosive process streams.
The largest industrial application for fiber bed mist eliminators is the removal of sulfuric acid from acid-manufacturing streams. Currently, mist eliminators largely employ fiberglass fiber beds in this application. Some fiberglass exhibits excellent chemical resistance to corrosive acidic mists and successfully operates over a wide temperature range. However, fiberglass slowly dissolves over time due to hydrolytic attack from water. Fiberglass is also attacked by ammonia dissolved in water and consequently fails in ammonia-water process streams. Some fiberglass is not effective under caustic (pH&gt;9) conditions. Thus, fiberglass can be unattractive for many fiber bed filtration applications.
There is a need for fiber bed separation media that increases the separation efficiency of a fiber bed mist eliminator, while lowering the operating costs of the separation. An improved fiber bed material should also accommodate a wide range of process stream constituents, including acids, caustics and other corrosives. A fiber bed separation media is needed that is further resistant to the hydrolytic effects of water and which lends itself to surface area-improving designs.